The present invention constitutes an improvement over the bidirectional belt drive assembly described in U.S. patent Application Ser. No. 692,484 filed June 3, 1976, filed in the name of R. Howard et al and assigned to the assignee of the present application and abandoned in favor of continuation application Ser. No. 28,318, filed Apr. 9, 1979. The apparatus disclosed therein is comprised of a closed loop belt entrained about a pair of pulleys and tensioned so as to provide upper and lower substantially linear belt runs. A carriage assembly is mounted for slidable movement along a pair of guide rods extending substantially parallel to said belt runs. A print head is mounted upon the carriage for effecting printing upon a paper web arranged to pass along a platen which is aligned substantially parallel to said guide rods.
In order to cause printing in a desired direction, a first, one of a pair of solenoid control means is energized to rotate a jam cleat into wedging engagement with one run of said belt, the jam cleat being urged against said belt to clamp said belt between the wedging member and the surface of a backing means. The angular orientation of the wedging member cooperates with the direction of movement of the belt to cause the wedging member to be wedged against the belt and thereby enhance the clamping action to effect abrupt acceleration of the carriage assembly.
Movement of the carriage assembly in the reverse direction is accomplished by de-energizing the aforementioned first solenoid control means to release its jam cleat assembly and energizing the remaining solenoid control means to cause its associated jam cleat assembly to become wedged against the opposite run of said belt. The belt is driven at a constant rate by a constant speed motor whose output shaft rotates the drive pulley in one direction at a constant angular velocity, the opposite pulley being an idler pulley driven to rotate by virtue of the belt entrained therearound.
With printers of this type it is extremely important to be able to accelerate from a standstill position up to the constant velocity desired for printing as rapidly as possible. This objective is basically achieved by the above-mentioned wedging action. However, it is extremely important to provide adequate positive engagement between the jam cleat assemblies and the belt in order to clamp the belt and it is thereby important to provide an engaging surface of sufficient surface area to prevent undue wearing of both the jam cleat assembly wedging portion and the belt, while at the same time retaining the highly desirable wedging configuration.
It is further highly desirable to provide a carriage assembly which, while having all of the above attributes, has a low mass in order to facilitate its rapid acceleration.
Also the carriage taught in the aforementioned pending application requires that the carriage be moved relative to the platen by means of a complex eccentric mounting for adjusting the position of the print wire tips relative to the platen thereby complicating the carriage assembly.